This invention relates to a siloxane-based lubricant composition and in particular, to such a composition which is useful for coating a tire curing bladder as part of a tire manufacturing operation.
In the manufacture of a pneumatic rubber vehicle tire, shaping of the tire is accomplished by inflating a rubber bag, or curing bladder, inside a green tire carcass thereby forcing the tire against the mold surface. Generally, there is considerable relative movement between the outer surface of the bladder and the inner surface of the green tire during the expansion phase of the bladder prior to fully curing the tire. Similarly, there is also considerable relative movement between the bladder and the cured tire after the tire has been molded and vulcanized and the bladder has been collapsed and stripped from the inner surface of the tire in order to reduce friction between the bladder and the inside of the tire and provide optimum slip of the bladder during the shaping process when the raw tire and bladder are in relative movement with respect to each other.
A tire curing bladder lubricant, intended to be applied as a coating on the surface of the bladder, is disclosed in U.S. Pat. No. Re. 32,318. The lubricant includes (A) from about 20 to about 40 parts by weight of a polydimethylsiloxane having a viscosity of from about 12 to about 28 million centistokes at 25.degree. C., (B) from about 35 to about 70 parts by weight of at least one silane selected from (i) a methyl hydrogen silane having a viscosity of from about 20 to about 40 centistokes at 25.degree. C., (ii) a dimethyl hydrogen silane having a viscosity of from about 80 to about 120 centistokes at 25.degree. C. and (iii) a methyltrimethoxy silane, (C) optionally, from about 3 to about 2 parts weight of a metal salt of an organic acid, (D) from about 10 to about 25 parts by weight of one or more surfactants and (E) from about 500 to about 1500 parts by weight of water to provide an emulsion or dispersion. The patent indicates a preference for a hydroxyl-capped polydimethylsiloxane for (A) and a mixture of methyl hydrogen silane and dimethyl hydrogen silane as (B) in the foregoing lubricant.
While the lubricant of U.S. Pat. No. Re. 32,318 containing methyl hydrogen silane has been found to be effective for its intended purpose, it has been observed that during storage and/or use in a tire curing operation, potentially hazardous hydrogen gas is given off as a result of the decomposition of the methyl hydrogen silane levels.